I've gone to AZ for 2-3 months for over 10 years. Several times I've looked into a rental for the whole duration, but the cost is outrageous. There aren't, it seems, discounts for longer rentals.
So if you can manage the way the above poster does, just taking the 9.99 weekend days, that's the cheapest. Phoenix being spread out like it is, though, I wanted the car on my own schedule. So either toaded it, or drove it down. Now I just leave an extra car there, and it's my best solution (doubt it's the cheapest, though!)